At present, a wide variety of IMDs are commercially released or proposed for clinical implantation. Such IMDs include implantable cardiac pacemakers as well as implantable cardioverter/defibrillators (ICDs) providing automatic cardioversion/defibrillation, anti-tachycardia pacing and bradycardia pacing functions of one or more heart chamber, drug delivery pumps, cardiomyostimulators, cardiac and other physiologic monitors, nerve, muscle, and muscle stimulators, e.g., sacral and spinal nerve stimulators and deep brain stimulators, cochlear implants, artificial hearts, etc.
The cardiac pacemakers, ICDs, and the various tissue, organ and nerve stimulators typically comprise an implantable pulse generator (IPG) comprising a hermetically sealed enclosure or “can” or housing and connector header combined with one or more elongated electrical medical lead coupled to the connector header. Certain implantable hemodynamic monitors also comprise a hermetically sealed enclosure or “can” or housing and connector header combined with one or more elongated electrical medical lead coupled to the connector header. Other implantable monitors comprise a hermetically sealed enclosure or “can” or housing and a sensor header that only supports a sensor, e.g., an EGM sense electrode. Such connector headers and sensor headers are formed of dielectric materials having dielectric properties. For convenience, unless specifically referred to, such dielectric connector headers and sensor headers are collectively referred to hereafter as “headers”. Such hermetically sealed housings are typically formed of a conductive bio-compatible metal, although proposals have been made to form hermetically sealed housings of a non-conductive, bio-compatible polymeric or ceramic.
Such IPGs and monitors are intended to be implanted subcutaneously in a surgically prepared pocket and are designed and fabricated to be as thin and light as possible to be cosmetically unobtrusive and to avoid movement-related complications. The hermetically sealed housings and headers of such IPGs and monitors are specified as having a thickness, a height, and a width that define a displaced volume and a weight in grams. It is sought in any given design to minimize all of these specifications. The height and width are related to corresponding height and width dimensions of relatively large surface area, opposed major sides of the housing and height and width dimensions of the header. The thickness is specified to be substantially less than the height and width to facilitate implantation.
The opposed major sides can be shaped having substantially circular, oval or rectilinear outlines and can have relatively straight and curved side edge sections. The opposed major sides are typically planar and disposed substantially in parallel, although the major sides may be bowed, convex or concave or otherwise contoured to some degree to conform to a particular implantation site. The opposed major sides are typically supported and joined together at their side edges by a mutual sidewall extending between them and having a sidewall width substantially defining the thickness of the hermetically sealed housing. The mutual sidewall extends through a number of sidewall turns or corners depending on the circular, oval or rectilinear outline or combination of such outlines of the opposed major sides. Generally speaking, such housings are referred to as “prismatic”.
The header is typically molded from of a relatively hard, dielectric, non-conductive polymer having a header thickness corresponding to the housing thickness and a header mounting surface that conforms to and is mechanically affixed against a mating sidewall mounting surface. The header has a header height measured in a direction extending away from the housing sidewall mounting surface and a header width measured in a direction extending along the housing header mounting surface in the width dimension of the housing. An exemplary monitor header and housing formed in this manner is disclosed in commonly assigned U.S. Pat. No. 5,851,221. An exemplary IPG header and housing formed in this manner are disclosed in commonly assigned U.S. Pat. No. 4,182,345. Other examples are found in further patents referenced herein.
The hermetically sealed housing encloses a battery providing power to electronic circuitry and associated components for providing therapies and/or monitoring and detecting conditions of the body. A sensor header supporting a sense electrode or other sensor or a connector header adapted to make connection with a proximal connector assembly of an elongated electrical medical lead or catheter is physically attached to a rectilinear housing header mounting section of the common sidewall that is typically, although no necessarily, planar and straight. One or more electrical feedthrough is mounted to the sidewall mounting surface to extend one or more feedthrough pin from the electronic circuitry into the connector header or header to make an electrical connection with one or more connector element or sensor or sense electrode supported within the connector or sensor header.
It has become common to provide a communication link between the hermetically enclosed electronic circuitry of the IMD and an external programmer or monitor or other external medical device (herein an EMD unless otherwise identified) in order to provide for downlink telemetry (DT) transmission of commands from the external device to the IMD and to allow for uplink telemetry (UT) transmission of stored information and/or sensed physiological parameters from the IMD to the EMD. As the technology has advanced, IMDs have become ever more complex in possible programmable operating modes, menus of available operating parameters, and capabilities of monitoring increasing varieties of physiologic conditions and electrical signals which place ever increasing demands on the programming system. Conventionally, the communication link between the IMD and the EMD is by encoded RF transmissions between an IMD RF telemetry antenna and transceiver and an EMD RF telemetry antenna and transceiver.
The telemetry transmission system that evolved into current common use relies upon the generation of low amplitude magnetic fields by current oscillating in an LC circuit of an RF telemetry antenna in a transmitting mode and the sensing of currents induced a closely spaced RF telemetry antenna in a receiving mode. Short duration bursts of the carrier frequency are transmitted in a variety of telemetry transmission formats. In the MEDTRONIC® product line, the RF carrier frequency is set at 175 kHz, and the RF telemetry antenna of the IPG or monitor is typically coiled wire wound about a ferrite core. The EMD is typically a programmer having a manually positioned programming head having an external RF telemetry antenna. The ferrite core, wire coil, RF telemetry antenna is not bio-compatible and can be damaged by body fluids that can penetrate the connector header, and therefore the antenna must be located inside the hermetically sealed housing. The typically conductive housing adversely attenuates the radiated RF field and limits the data transfer distance between the programmer head and the IMD RF telemetry antennas to a few inches.
The current MEDTRONIC® telemetry system employing the 175 kHz carrier frequency limits the upper data transfer rate, depending on bandwidth and the prevailing signal-to-noise ratio. Using a ferrite core, wire coil, RF telemetry antenna results in: (1) a very low radiation efficiency because of feed impedance mismatch and ohmic losses; 2) a radiation intensity attenuated proportionally to at least the fourth power of distance (in contrast to other radiation systems which have radiation intensity attenuated proportionally to square of distance); and 3) good noise immunity because of the required close distance between and coupling of the receiver and transmitter RF telemetry antenna fields.
These characteristics require that the IMD be implanted just under the patient's skin and preferably oriented with the RF telemetry antenna closest to the patient's skin. To ensure that the data transfer is reliable, it is necessary for the patient to remain still and for the medical professional to steadily hold the RF programmer head against the patient's skin over the IMD for the duration of the transmission. If the telemetry transmission takes a relatively long number of seconds, there is a chance that the programmer head will not be held steady. It is necessary to restart and repeat the uplink telemetry transmission if the uplink telemetry transmission link is interrupted by a gross movement.
In U.S. Pat. No. 4,785,827, and commonly assigned U.S. Pat. No. 5,470,345, the metal container typically used as the hermetically sealed housing of the IMD is replaced by a hermetically sealed ceramic container. The wire coil antenna is still placed inside the container, but the magnetic H field is less attenuated. It is still necessary to maintain the IMD and the external programming head in relatively close proximity to ensure that the H field coupling is maintained between the respective RF telemetry antennas.
Many proposals have been advanced for eliminating the ferrite core, wire coil, RF telemetry antenna and substituting alternative telemetry transmission systems and schemes employing far higher carrier frequencies and more complex signal coding to enhance the reliability and safety of the telemetry transmissions while increasing the data rate and allowing telemetry transmission to take place over a matter of meters rather than inches and do away with the programmer head (referred to as “far field” telemetry in certain cases). A wide variety of alternative IMD telemetry antennas mounted outside of the hermetically sealed housing within the connector header or a further header formed of a dielectric material and coupled to the telemetry transceiver within the hermetically sealed housing via a feedthrough mounted in the housing sidewall have been proposed.
In one approach, it is proposed that the elongated wire conductor of one or more electrical medical lead extending away from an IPG or monitor be employed as an RF telemetry antenna in U.S. Pat. Nos. 5,058,581 and 5,562,713 and in U.S. Patent Application Publication No. 2002/0065539, for example. A modest increase in the data transmission rate to about 8 Kb/s is alleged in the '581 and '713 patents using an RF frequency of 10–300 MHz. In these cases, the conductor wire of the medical lead can operate as a far field radiator to a more remotely located programmer RF telemetry antenna. Consequently, it is not necessary to maintain a close spacing between the EMD and IMD RF telemetry antennas or for the patient to stay as still as possible during the telemetry transmission. As noted in Publication No. 2002/0065539, the lead conductor wire can be selectively tuned in length to provide optimal theoretical frequency characteristics.
However, using the medical lead conductor as the RF telemetry antenna has several disadvantages. The radiating field is maintained by current flowing in the lead conductor, and the use of the medical lead conductor during the RF telemetry transmission may conflict with sensing and stimulation operations. RF radiation losses are high because the human body medium is lossy at higher RF frequencies. The elongated lead wire RF telemetry antenna has directional radiation nulls that depend on the direction that the medical lead extends, which varies from patient to patient. These considerations both contribute to the requirement that uplink telemetry transmission energy be set artificially high to ensure that the radiated RF energy during the RF uplink telemetry can be detected at the programmer RF telemetry antenna. Moreover, not all IMDs have lead conductor wires extending from the device.
A further U.S. Pat. No. 4,681,111 suggests the use of a stub antenna associated with the header as the IMD RF telemetry antenna for high carrier frequencies of up to 200 MHz and employing phase shift keying (PSK) modulation. The elimination of the need for a VCO and a bit rate on the order of 2–5% of the carrier frequency or 3.3–10 times the conventional bit rate are alleged. Partially shielded stub antennas for RF telemetry that project away from IMD header are disclosed in commonly assigned U.S. Pat. Nos. 6,169,925 and 6,240,317. These outwardly extending, elongated stub antennas present implantation problems since they must be extended subcutaneously for a distance away from the IMD enclosure, and share other disadvantages of use of medical electrical lead conductors as antennas.
It is proposed in commonly assigned U.S. Pat. No. 5,861,019 to form a microstrip RF telemetry antenna on or within the exterior surface of an IMD housing that is formed either of a conductive metal or of a non-conductive dielectric material. The microstrip antenna is formed of an electrically conductive radiator patch layer that is laminated upon an exterior facing side of a dielectric substrate layer of relatively constant thickness. A conductive ground plane layer is formed on the opposite side of the dielectric substrate layer to extend parallel to and at least coextensively with the radiator patch layer. The radiator patch layer is coupled to the transceiver circuitry within the IMD housing by a feedthrough extending through the dielectric substrate layer, the ground plane layer and the IMD housing side. If the IMD housing is conductive it may form the ground plane layer over which the dielectric substrate layer and the radiator patch layer are formed through deposition or other techniques. If the IMD housing is formed of a suitable non-conductive dielectric material, the ground plane layer is formed on an interior surface thereof and the radiator patch layer is formed on an exterior housing surface thereof, preferably by deposition techniques. The ground plane layer may be recessed to form a cavity backed ground plane that receives the dielectric layer and radiator patch layer within the cavity. The exterior surfaces of the radiator patch layer, the dielectric layer and any exposed surface of the ground plane layer may be electrically insulated by a radome layer which is formed of the same dielectric as the dielectric layer. While this approach has merit, it requires complex manufacturing techniques and expense. Moreover, surgeons could mistakenly implant the IMD housing so that the microstrip antenna is disposed inward, significantly reducing the telemetry range with an external medical device.
It is generally preferable to provide an IMD RF telemetry antenna that performs as required but does not unduly increase the thickness of the IMD, the number of components that have to be implanted or assembled in use, and the cost of manufacturing. It has been proposed that one way of realizing these preferences is to support the IMD RF telemetry antenna on or within the IPG connector header.
It is suggested in U.S. Pat. Nos. 5,342,408 and 5,730,125 that an IMD RF telemetry antenna may be located in an IMD connector header and coupled to the transceiver via at least one feedthrough through the enclosure wall extending directly into the IMD connector header. Moreover, a relatively large, air core, RF telemetry antenna has been embedded into the thermoplastic connector header material of the MEDTRONIC® Prometheus programmable IPG. A proposal to place an RF telemetry antenna coil either in the IMD connector header or in a second header mounted to the hermetically sealed enclosure a distance away from the connector header is made in U.S. Pat. No. 5,313,953. Similar proposals appear in U.S. Patent Application Publication Nos. 2002/0123776 involving use of coiled wire antennas. In these approaches, attention has been directed to making the IMD RF antenna fit within the IMD existing connector header as is also suggested in further U.S. Pat. Nos. 6,434,429 and 6,470,215. Similar proposals for incorporating an EMI detecting wire strip antenna within the header are set forth in U.S. Pat. No. 5,697,958,
However, it remains desirable to keep the connector header as small as possible to avoid increasing the overall dimensions and weight of the IMD that must be subcutaneously implanted. Consequently, there is little room for a wire strip or coil antenna having sufficient length to be confined within the typical header that is mounted to the sidewall of the hermetically sealed housing.
It is therefore proposed to extend the RF telemetry antenna from the header around the perimeter of the hermetically sealed enclosure in certain embodiments of the above-referenced '925 and '317 patents (see FIG. 7) and in U.S. Pat. No. 6,456,256. An elongated wire antenna is supported within an extended dielectric header spaced from the housing sidewall and extending from a feedthrough pin of an antenna feedthrough mounted in the sidewall around at least one corner of the sidewall and then through a predetermined length of the sidewall to an antenna wire free end at the termination of the extended header. The header is a connector header for an IPG or a monitor having connector elements and connector bores adapted to receive one or more electrical medical leads. The antenna feedthrough is mounted in the housing sidewall in the vicinity of the connector elements.
In the embodiment of the above-referenced '925 and '317 patents, the antenna wire extends outward away from the antenna feedthrough alongside the connector bore(s) toward the outer surface of the connector header. The antenna wire is curved to extend along the width of the connector header to a corner thereof overlying a corner of the housing sidewall and is bent to extend around the corner and along a side section of the sidewall embedded within the extended connector header. Thus, an elongated stub antenna is wrapped around the perimeter of the hermetically sealed housing in these embodiments so that it is fixed in position rather than extending freely away from the connector header within a dielectric coating as shown in other embodiments of the above-referenced '925 and '317 patents.
In the above-referenced '256 patent, the opposed major surfaces of the hermetically sealed housing have a ovaloid edges that are joined by a curved side. The antenna wire extends from the feedthrough pin supported within a similarly curved extension of the connector header extending over the curved sidewall. The curved connector header extension and curved antenna wire extend along the housing sidewall almost to the connector bore openings of the connector header.
The available space within a connector header adjacent the connector elements and the associated connector bores, feedthroughs, electrical conductors and mechanical fixing elements is relatively limited, and it is disadvantageous to locate an additional RF antenna feedthrough in proximity therewith. Accommodating the antenna feedthrough and making the electrical connection between the antenna feedthrough pin and the antenna wire when the connector header is mounted to the sidewall mounting space may be difficult in this crowded environment.
In these approaches, the conductive antenna wire is embedded within the dielectric polymer of the connector header spaced apart from the hermetically sealed sidewall, which is electrically connected to function as a grounding plane. This displacement distance between the antenna wire and the hermetically sealed grounding plane is relatively fixed through the length of the extended connector header. In the above-referenced '256, '925 and '317 patents, full advantage is not taken of the height of the connector header in the section supporting the connector elements and connector bores to displace a section of the antenna wire at a further displacement distance.
It remains desirable to provide a telemetry antenna for an IMD that eliminates drawbacks associated with the IMD telemetry antennas of he prior art. As will become apparent from the following, the present invention satisfies this need.